Semiconductor devices are constructed of patterned layers of electrically conductive, non-conductive and semi-conductive materials stacked over a silicon wafer. The layers of material are successively deposited on the wafer and etched into predefined patterns to form individual component structures within the particular device being fabricated. The manufacturing process often includes deposition and/or etch steps in which semiconductor wafers are processed in a group or "batch." Batch processing tools include barrel reactors, diffusion furnaces and the like, wherein each batch of wafers may be subjected to various chemical, mechanical or electrical processes.
The rate of production of conventional batch processing tools, such as those used for plasma etching and chemical vapor deposition, is often limited by the diffusion of reactant gases or reaction products to the center of the wafers. This is especially true in tube shaped reactors which hold a batch of wafers closely spaced and parallel to one another. In such conventional reactors, which are operated at a near constant pressure, the gaseous reactants and reaction products diffuse through the small space between the wafers. The diffusion of the reactants and reaction products to the center of the wafers is typically the slowest step in the deposition or etch process. In addition, the chemical and mechanical processes through which materials are deposited on or etched off the surfaces of a wafer occur more slowly at the center of the wafer than at the edge of the wafer. Therefore, the deposition or etch profile tends to be non-uniform across the surface of the wafer. That is, more material is deposited on or etched from the edge of the wafer than is deposited on or etched from the center of the wafer.